It has been proposed to transport telecommunications signals over a power line. International Patent Application WO 94/09572 A1 describes such a network. Delivering a telecommunications service in this manner is attractive as it overcomes the need for installing cabling to each subscriber, which is one of the greatest costs in providing a new telecommunications network. Existing power lines are used to carry the telecommunications signals to subscribers.
There are a number of different methods for carrying telecommunications signals over a power line. One method modulates an RF carrier with the telecommunications information and couples the modulated RF carrier on to the power line. One of the potential problems with transporting RF signals over a power line is that of unwanted radiation of RF energy from the power line and other parts of the electricity distribution network. This is because the electricity distribution network was not designed to carry RF signals.
One significant point where radiation can occur is at the substations where electricity is transformed from high voltage (eg 6.6 or 11 kV) to low voltage (400 V). Substations have busbars which are typically mounted as a grid array on the substation wall. The busbars are shielded from view but often are electrically unscreened. This is because screening is considered unnecessary at the 50 Hz mains frequency. At RF frequencies the busbar array functions as an antenna, radiating the RF signals which it receives via the distribution cables into the surrounding area. This is undesirable as it causes interference with equipment operating at these frequencies. This radiation may also violate regulations on Electromagnetic Compatibility (EMC).
One of the solutions to minimise radiation from the busbars is to screen the busbar array, or to screen the entire substation building. Some modern substations are equipped with metal casing around the busbars. However, the majority of substations are unscreened brick structures. It is undesirable to renovate all of these structures to improve their screening as it increases the cost of providing a telecommunications service over the network.
An alternative to the radiation problem is to restrict the power at which the RF signals are transmitted over the network, such that radiation occurring at substations falls within acceptable limits. This causes problems with subscribers' equipment, particularly to those subscribers furthest from the point at which RF signals are injected onto the network. Subscriber equipment needs an acceptable signal to interference ratio in order to detect the wanted RF signals. With considerable interference on the network, this demands a reasonably high transmit power.
The problem of radiation at the substation is compounded by the fact that RF signals are usually injected onto the distribution network adjacent to the substation. The reason for injecting at this point is because one base station can easily be coupled to each of a group of low voltage (400 V) cables which all converge at the substation.